The OFDM transmission system is adopted in a digital terrestrial broadcasting system in Japan and Europe. In an ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) system in Japan and a DVB-T (Digital Video Broadcasting-Terrestrial) system in Europe, both of which adopt the OFDM transmission system, pilot signals having a known amplitude and phase are scattered in a sub carrier for a frequency domain. These pilot signals are called scattered pilot signals (hereinafter, referred to as SP signal). Also, in Europe, a DVB-H (Digital Video Broadcast-Handheld) system based on the DVB-T system is standardized for a portable receiver. In the DVB-H system, pilot signals having a known amplitude and phase are scattered and inserted in a sub carrier for a frequency domain.
The following describes an arrangement of SP signals, with reference to FIG. 1. FIG. 1 shows the arrangement of the SP signals. The SP signals are not transmitted by each sub carrier, but are transmitted by a sub carrier in which a sub carrier index k satisfies k=3 (n mod 4)+12p, where n indicates a symbol index (mod indicates a modulus operator, and p is an integer). In detail, as shown in FIG. 1, the SP signals are repeated in a cycle of 4 symbols and shifted by 3 sub carriers for each symbol.
Each SP signal is transmitted after being modulated into a binary signal by a transmitter, based on a predetermined pattern determined according to a sub carrier position of the SP signal. A receiver adjusts a phase of the SP signal, and then performs interpolation in a time axis direction (symbol direction) and a frequency axis direction (carrier direction) using a LPF (Low Pass Filter) to estimate channel characteristics for all sub carriers. Then, the receiver performs complex division on data signals by the estimated channel characteristics, to obtain data signals that are equalized corresponding to an effect of a channel.
However, when noise is superimposed on a SP signal itself and a noise component is large, because the SP signal is used for estimating channel characteristics, an estimation error of the channel characteristics becomes large.
The following method can be used to solve the above problem.
After performing a FFT (Fast Fourier Transform) on a received OFDM signal, SP signals are extracted. Then, an IFFT (Inverse Fast Fourier Transform) is performed on the extracted SP signals to obtain a signal in a time domain, and a signal component of a predetermined duration of the signal after the IFFT is multiplied by a rectangular window. As a result, a noise component is removed from the SP signals (refer to a patent document 1, for example). Then, the FFT is performed on the signal after the noise removal, and interpolation processing and the like are performed to estimate channel characteristics for all of sub carriers.
Although an object is different from the above method, the following is a prior art that performs the similar processing as the above method.
SP signals are phase adjusted and interpolated in the symbol direction using the LPF, and the IFFT is performed on a data group in a same symbol after the interpolation. A portion of data of the data group after the IFFT is set to zero, and the FFT is performed on the data to obtain the channel characteristics of all of sub carrier positions (refer to patent documents 2 and 3, for example). The purpose of this prior art is to minimize the effect of a ripple (pulsation) of a pass band and attenuation characteristics caused by the LPF used in the carrier direction. Therefore, the object of the prior art is reducing the above influence. However, the object of the prior art is not removing noise from SP signals based on which channel characteristics are estimated.
Patent Document 1: Japanese Patent Publication No. 3044899
Patent Document 2: Japanese Published Patent Application No. 2002-64413
Patent Document 3: Japanese Published Patent Application No. 2003-101503